Installation of roof windows. Learn the rules and work sequence

Window installation is carried out by roofers. The most important steps are to level the roof window, place insulation material around the frame, install a vapour-permeable collar to protect the insulation from moisture and a flashing to drain water from rain or snow.

Regardless of whether the window is installed in the roof at the stage of covering or later, it is most important to carefully seal its contact with the roof layers. Roof windows may be designed to fit into the rafter framing in a specific location, but on site they need to be matched to the roofing material chosen. It is not only a matter of choosing the right flashing, which is chosen according to the height of the roofing, but also of fitting it properly into the roofing. A row of tiles of full height has to fit under the window in order to fix it stably to the slope. Before installing a roof window, roofers have to cover the rafter framing with roofing membrane or rigid boarding and fix a grid of counter-battens and battens. If the roof is tiled, the window installation can also take place at any later date, as it is very easy to remove. In the case of metal sheets, it is better to install the windows before tightening the sheets to the grid.

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Roof window size and frame

Roof windows are manufactured in widths similar to the rafter spacing most commonly used by designers in roof trusses. In new single-family houses the rafter spacing is usually between 90 and 100 cm. This means that windows 55, 66, 78 or even 94 cm wide can be installed without any interference in the structure. Wider windows – 114 or 134 cm – require cutting out a fragment of the rafter and inserting exchanges, i.e. beams perpendicular to the cut rafter, which will transfer the load to adjacent full-length rafters. If you choose a window which is 4-6 cm less wide than the distance between rafters, its assembly will be quick and uncomplicated. Narrower windows require fixing auxiliary rafters to the rafters on one or both sides of the window.

Installation of windows to battens or rafters

Roof windows are fixed using mounting brackets sold with the window kit. They are screwed to the 4-5 cm wide battens or directly to the rafters. In new houses, in which a usable attic is planned, the rafter spacing is usually selected in such a way that a window can be easily installed between them. The window is installed at the same time as the roofing, before the attic is finished on the inside and insulated.

Joining windows to the roof membrane

Before installing windows, the roof should already be covered with a vapour-permeable roofing membrane or other pre-insulation. There is usually a section of the final roof covering without a section around the window itself. No roofing battens are nailed in place of the planned window. If they have already been installed, the battens are cut out. The roof membrane is cut in the place of the planned window, leaving enough space to turn it up over the battens.

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Heat around the frame

Reduction of heat escape through the roof window is achieved not only by proper selection of the depth of its installation but also by careful insulation of the frame. Additional insulation may be laid both at the level of battens and above them. Manufacturers offer various sets of thermal insulation. If you opt for the warmest windows, designed for passive houses, you can buy those with a factory-fitted thermal insulation frame around the perimeter of the window. It improves window insulation by 9-15% and effectively prevents condensation on the pane and frame. Other manufacturers offer insulation sets, consisting of thermal insulation (polyurethane foam frame or braids made of natural sheep wool) and insulation protecting it from both sides – anti-moisture on the outside and vapour-proof on the loft side.

Levelling the roof window

The frame should be very carefully levelled on the battens, otherwise the window will not open properly. To check the final positioning, place the sash in the frame for a moment and check that the gaps between the window elements are parallel. Unevenness in the frame mounting can be levelled with shims under the mounting angles. The space of several centimetres between the frame and battens is insulated around the window with a foamed polyethylene insulation frame matched to the size of the window. Next, a ready-made damp-proof membrane similar to accordion or traditional damp-proof membrane is installed around the window to seal with the roof membrane.

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Installing the flashing

The final step in the installation of the roof window is the installation of the flashing, which should be matched to the type of roofing material and its height. On a tiled or tile roof, the flashing must have a special apron which is carefully moulded to fit the cut tiles. At the top it has a bar which supports high elements of the roofing and gutters for water drainage.

Once the insulation on the roof has been completed, we move on to internal installation activities, such as laying vapour-barrier foil around the window, insulating the window recess, and actually shaping the recess and finishing it with plasterboard. These activities can also be made easier by using ready-made materials adjusted to the size of the window.


Window recess

When installing the window it is extremely important to make the window recess properly. Making it can be time consuming and sometimes due to the lack of time or care it is not perfect. Therefore a good solution are ready-made products that make the work easier e.g.: a steel frame for making a recess or a ready-made window recess.

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